1. Field of the Invention
The present invention relates to an apparatus for detecting a transported sheet in, for example, printers, facsimiles, or other devices that print on transported sheets, and more specifically relates to an apparatus for detecting a transported sheet so as to control the printing position on the transported sheet.
2. Description of the Related Art
In printers and like image forming devices, the position of the sheet transported by the transport means must be detected, and the print timing must be controlled on the basis of the detection results so as to print at a specific position on the sheet. Configurations such as that disclosed in Japanese laid Open Pat. application No. 59-57778 are known as sheet position detection apparatus used for the purpose described above. The sheet detection apparatus described in the aforesaid patent application provides a stationary roller disposed in the sheet transport path and a pressure roller for applying pressure to said stationary roller, such that the slight movement of the pressure roller induced each time a sheet passes medially to said pressure and stationarv rollers is mechanically amplified by means of an oscillating arm and detected.
On account of the mechanical amplification and detection of the slight movement of the pressure roller by means of an oscillating arm, the aforesaid apparatus requires laborious adjustment of the mechanical apparatus which makes maintaining a high precision difficult, and the apparatus also has the disadvantage of being enlarged in overall size.
Japanese Examined Open Pat. application No. 61-25283, on the other hand, discloses a feed detection apparatus in the sheet transport portion of a paper currency operated machines. The apparatus provides a stationary roller, a movable roller for applying pressure to said stationary roller, a light-emitting portion which projects light rays toward the transporting surface of said movable roller, a photoreceptor portion having a plurality of photoreceptors to receive the light reflected from the transporting surface of the aforesaid movable roller, and a plurality of slits disposed anteriorly to each respective photoreceptor. When a paper sheet passes between the rollers, the movable roller is moved only a distance corresponding to the thickness of said paper sheet, and the direction in which the light reflected from the surface of the movable roller is reflected changes accordingly. The aforesaid plurality of photoreceptors are disposed in positions to receive reflected light when no paper sheet is between the rollers, when one sheet is between the rollers, and when two stacked sheets are transported, and the counting of the transported sheets and detecting of the stacked sheets are accomplished by differentiating which of the photoreceptor receives the reflected light.
Although the apparatus disclosed in the aforesaid Japanese Examined Pat. application No. 61-25283 has a compact design because the movement of the movable roller is detected by the photosensor directly without mechanical amplification by some means, there is a danger of detection errors caused by irregular reflection due to the surface condition of the movable roller or soiling or damage to the surface thereof.